Explain why the presence of nitrates in drinking water is considered harmful to human health, and state one agricultural practice that causes nitrates to enter water supplies.

Cambridge International IGCSE Chemistry (0620) — 10.1 Water · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Nitrates in drinking water are converted in the body to nitrites, which bind to haemoglobin and reduce its ability to carry oxygen. This can lead to methemoglobinaemia, especially in infants, causing the characteristic blue‑baby syndrome.

One agricultural practice that introduces nitrates into water supplies is the excessive use of artificial fertilisers, which are washed off the fields by rain into rivers and groundwater.

Examiner tips

  • Use the term ‘methemoglobinaemia’ or ‘blue‑baby syndrome’ to show depth of knowledge
  • Show the conversion step: nitrate → nitrite
  • Mention infants/young babies as the most vulnerable group
  • Include the specific practice: excessive artificial fertiliser use

Common mistakes

  • Failing to mention the conversion to nitrites
  • Using ‘oxygen transport’ instead of ‘haemoglobin’
  • Omitting the vulnerable group (infants)

Mark scheme (4 marks)

  1. Nitrates in drinking water can be converted to nitrites in the body / gut
  2. Nitrites reduce the ability of haemoglobin / blood to carry oxygen
  3. This is particularly dangerous for infants / young babies (blue baby syndrome)
  4. Nitrates enter water supplies due to the use of (artificial / excess) fertilisers on agricultural land (which are washed into rivers / groundwater by rain)

Key terms in this question

nitrates

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